Fiber lasers and amplifiers are promising pulse source candidates for industrial applications, due to their unique simplicity of construction. Large core fiber amplifiers and specifically large core diffraction limited multi-mode amplifiers (as described by M. E. Fermann and D. Harter, ‘Single-Mode Amplifiers and Compressors based on Multi-Mode Optical Fibers’, U.S. Pat. No. 5,818,630) (incorporated by reference herein) enable the amplification of optical signals to levels where applications such as micro-machining and laser marking become possible. See A. Galvanauskas et al., U.S. patent application Ser. No. 09/317,221 (incorporated by reference herein). Since laser marking and micro-machining are dependent on the supply of high peak power pulses, it is advantageous to use such fiber amplifiers for the amplification of nanosecond regime (ns) pulses, as supplied, for example, by micro-chip lasers or general Q-switched sources. In conjunction with large-core fiber amplifiers, these ns pulse sources can be amplified to pulse energies in the hundreds of microjoules (μJ). Such fiber amplifier systems can operate as direct replacements for Nd-based solid state lasers in any micro-machining or marking application.
The use of diffraction limited fiber multi-mode amplifiers allows a significant improvement in the power density to be delivered to a target compared to non-diffraction-limited multi-mode amplifiers as described for example in B. Desthieux, Appl. Phys. Lett., vol. 63, No. 5, pp. 586-588 (1993)). Note that in this early work the use of electrically driven semiconductor lasers for the generation of short optical seed pulses to high power amplifier chains, albeit with a final multi-mode power amplifier, was described.
The use of electrically driven semiconductor lasers for amplifier chains was later reiterated by Grubb et al., U.S. Pat. No. 6,151,338. A variety of complex fiber amplifier implementations for use in micro-machining applications again reiterating the use of electrically driven semiconductor seed lasers was recently also suggested in U.S. Pat. No. 6,433,306 to Grubb et al. In U.S. Pat. No. 5,892,615 to Grubb, the use of bent single-mode amplifier fibers has been suggested; the difficulty in making such single-mode amplifiers has also been a limitation in such high power fiber amplifier systems.
Advanced micro-machining or micro-structuring applications are enabled by amplification of ultrafast optical pulses in fiber media. Ultrafast optical pulses are generally characterized by a pulse width of less than 50 ps; conveniently, chirped pulse amplification is implemented to enable the amplification of such pulses to the μJ-mJ energy range. Generally, chirped pulse amplification systems use a near bandwidth-limited seed pulse source, which is temporally stretched (i.e. chirped) in a pulse stretcher before amplification in a power amplifier. After amplification, the pulses are recompressed to approximately the bandwidth limit using a pulse compressor. In Ser. No. 10/606,829 (incorporated by reference herein) the present inventors disclosed how to make such systems reliable and compact by utilizing nonlinear fiber grating stretchers with single pass large-core polarization maintaining amplifiers. In this invention, we disclose how to utilize telecomm components similar to those in manufacturing an ultrafast source for this laser as is disclosed in IMRA Ref No. IM-108 (incorporated by reference herein). In Ser. No. 10/608,233 (incorporated by reference herein) we disclose how to down count the rep-rate utilizing lithium niobate modulators which are telecomm components. In this invention we will disclose practical problems and solutions utilizing AOM for these down counters. AOM for fiber amplifier systems were first disclosed in U.S. Pat. No. 5,400,350 (incorporated by reference herein). In this invention we disclose how to solve or utilize the problem of spatial dispersion caused by the AOM.
Recently, modular, widely tunable fiber chirped pulse amplification systems were described by M. E. Fermann et al., ‘Modular, High Energy Widely Tunable Ultrafast Fiber Source’, U.S. patent application Ser. No. 09/576,772, (incorporated by reference herein) which increase the spectral width of ultrashort pulses in amplifiers by parabolic pulse amplification. These pulses can then be recompressed to pulse widths shorter than the input pulses. In Ser. No. 10/437,057 (incorporated by reference herein) use of self-phase modulation in positive dispersion amplifiers with sources that have variable rep rates was described. Here we disclose how to utilize such systems in industrial FCPA systems. This leads to a careful match of the spectral components of the amplifiers and the oscillators as well as careful control to maintain output pulse properties. More recently parabolic amplification has been applied to FCPA systems in 2003/0156605.
Stabilization of gain against environmental variation is a key task for an industrial laser application. Most rare earth-doped fiber has a narrow absorption band where the wall-plug efficiency is high. In Er-doped fiber, usually a wavelength-locked 980 nm pump diode is used in order to avoid variation of the emission spectrum of the diode over temperature. (See, U.S. Pat. Nos. 5,563,732 and 6,335,944 B1, both incorporated by reference herein). The wavelength locking can be provided by partial feedback with a fiber Bragg grating. The temperature sensitivity of a silica based fiber Bragg grating is typically in the range of 10 pm/C, whereas a GaAs-based quantum well chip has sensitivity in the range of 0.3-0.5 nm/C. On the other hand, this concept is available if the absorption spectrum of the gain fiber is not widely varied over the required temperature range. Also this concept is appropriate for Er-doped fiber where the absorption spectrum is relatively stable over the industrial temperature range of 10-40 C. The pump wavelength and gain over temperature are passively set at a predetermined value.
These methods, however, have been only demonstrated for a pump diode with a single mode fiber coupler with the fiber Bragg grating written in a single mode fiber. Thus far, no embodiment of these methods has been described for high power diode bundles with a multimode fiber coupler. Difficulties lie in coupling cavities in stable coherence collapse regime with a multimode fiber pigtail and homogeneous Bragg grating fabrication in multimode fiber.
When effective wavelength locking is not available, an active regulation of the pump diode current or temperature is an alternative, by monitoring the pump wavelength. However, this solution faces a limit if the temperature-dependent variation of the gain fiber absorption is large, so that wavelength stabilization for the pump chip alone does not guarantee the required stable operation of the laser. An exact monitoring of the absorption spectrum of the gain fiber over temperature is required.
Thus the conventional concept described above is not applicable for stabilization of the gain fiber for stable output over a wide temperature range where: 1) wavelength locking using a device, e.g., a fiber Bragg grating, with a low temperature sensitivity in comparison with the pump device is not available, 2) the absorption spectrum of a gain medium has a large variation over temperature, so that a passively set wavelength does not provide the required stability of gain, 3) temperature sensing of the gain fiber is necessary for accurate gain stabilization.
The prior art device for extracting one or more pulses from a series of pulses produced by a high repetition-rate laser is an acousto-optic deflector. A system that utilizes such a pulse selector for micromachining is given in U.S. Pat. No. 6,340,806. This means works well with lasers that have very limited spectral bandwidths; typically lasers with pulse duration in excess of about a nanosecond. However, the wide spectral bandwidth of the output of an ultrashort-pulse laser results in the spatial separation of the various wavelength components of such a pulse when subjected to the dispersive Bragg grating of an acousto-optic deflector. This is a well known effect, and cavity dumping pulses from ultrafast oscillators or from regenerative amplifiers as shown in (T. B. Norris “Femtosecond Pulse Amplification at 250 kHz with a Ti:sapphire Regenerative Amplifier and Application to Continuum Generation: Opt. Lett. 17, pp. 1009-1011 (1992)) are performed by double passing the AOM with a curved mirror that brings all the components back to the same point. Thus the dispersion becomes corrected. This means could be used here, however the system would not be compact. The instant invention compensates for the dispersion, with the additional advantage of incorporating means to compress the chirped pulses to shorter duration.